Don't forget, Connor McLeod has competed in an elimination-style competition system which basically serves to prove who the best swordfighter among the immortals on Earth is. Most of these immortals has had the chance to train with multiple human master swordsmen, so each of their aptitude is bound to be pretty high. Cheating aside, McLeod has more or less proven, by winning the contest, that he is the best swordsman on Earth.